Add connection session process, event buffer and long-polling support

[kevinbader]

In order to support apps behind the Azure API Management Gateway with WebSockets disabled, we need to allow apps to "long-poll" for events. This means:

1. A client connects to RIG via a standard HTTP GET call (similar to SSE but not expecting chunked transfer encoding as a response)
2. As soon as one or more events are available, RIG sends them (as a list) as the HTTP reply to the client. That closes the HTTP connection.
3. After receiving the events (or a timeout), the client connects to RIG again, waiting for new events to become available.

In order to support this, we need a "Session" process that buffers incoming events. The ephemeral processes associated to incoming HTTP connections look up the session processes according to the client's JWT:

• if the client has a JWT, the token's jti identifies the session and with that also the session process. The Erlang process registry should be used to do the lookup, by registering session processes using their associated jti.
• if the client doesn't own a JWT, there is either no session process or the session process lives exactly as long as the connection process lives.

The session process handles

• inferring subscriptions from token and passed subscriptions parameter (parsing those values from the request should still be done in the request handlers)
• subscription refresh towards the filter processes
• receiving events from filter processes

Also, all session processes should be in a process group to support the new session API - to be implemented in #154 .

Going forward, the SSE and WS handlers should also rely on the session process (using the ring buffer, the SSE handler could even support SSE's "last-event-id" feature to re-send missed events). This is to be implemented in another issue.

[kevinbader]

Closed by #220